1. Technical Field
The present invention relates to a light source apparatus using laser light.
2. Description of the Related Art
Converting electricity into light more efficiently and emitting a greater amount of light than a light emitting diode (LED), a semiconductor laser is expected to be used as a light source for a projector, a headlight for an automobile, and other high-intensity white light sources.
Japanese Patent No. 4,054,594 (hereinafter also referred to as Patent Document 1) discloses a light source apparatus so configured that laser light emitted from a laser diode is focused onto a phosphor or fluorophore, which emits incoherent, spontaneously emitted light. The light source apparatus includes a visible light reflecting mirror having a parabolic reflection surface open toward a predetermined direction and a laser light reflecting mirror provided at the focal point of the visible light reflecting mirror and reflecting the laser light having passed through the phosphor back toward the phosphor.
Since laser light has intense power and a small spot size, its optical energy density is very high. Laser light could therefore damage human eyes. Light emitted from a typical semiconductor laser, which emits light having a small spot size, is focused into a microscopic spot on the retina and generates heat locally on the retina. Further, laser light having a visible wavelength and an eyeball or retina tissue on which the laser light is incident could undergo a biochemical reaction. In this case, the retina can be damaged even when the total amount of optical power is small.
In a white light source apparatus including a blue laser in combination with a phosphor, the phosphor is dispersed in a binder typically made of a silicone resin or any other material. The phosphor-containing resin typically has a smooth surface, which forms a reflection surface. Laser light directed toward the phosphor may not be absorbed by the phosphor but can be reflected by the surface of the binder resin. If the reflected light is focused through an optical system, the focused light may have a spot size equal to the spot size at the exit of the laser and could hence damage human eyes.
The invention has been made in view of the problems described above, and an object of the invention is to provide a light source apparatus having what is called an eye-safety function that ensures safety of human eyes, and another object of the invention is to provide a wavelength conversion structure used in the light source apparatus.
A wavelength conversion structure according to the invention is a wavelength conversion structure into which laser light is introduced and which outputs light having a wavelength different from the wavelength of the laser light. The wavelength conversion structure comprises a light guide formed of a light-transmissive member having a laser light incident port that allows the laser light to be introduced and a phosphor-containing layer that covers at least part of the surface of the light guide. The light guide has a light diffusing structure having asperities formed over the surface of the light guide except a laser light incident surface having the laser light incident port and a light reflecting film formed over the surface of the light guide along the asperities except the laser light incident port and the portion covered with the phosphor-containing layer.
A light source apparatus according to the invention comprises any of the wavelength conversion structures and a semiconductor laser provided adjacent to the wavelength conversion structure. The semiconductor laser is so disposed that a laser light exiting surface thereof faces the laser light incident port.
The wavelength conversion structure and the light source apparatus according to the invention are not dangerous to human eyes.